After ruling out other explanations, they believe that the signals could reveal a patch of liquid water, more than 12 miles across and about a mile beneath the ice.
Stephen Clifford, a scientist who works today at the Planetary Science Institute, first laid out the idea that there could be bodies of water beneath Mars' polar caps three decades ago.
A dose of realism: Liquid water in large quantities could be a real boon to any future Martian colonists.
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Following news of the findings, social media was understandably enthused, with some wondering what it might mean for the search for extraterrestrial life. "Those are not ideal conditions for life to form", Siebach said.
A new mission to Mars may have to wait, though. But, he cautions, "I'd say it's not quite the smoking gun". The reflection from a boundary involving liquid water is particularly strong. It also opens up the possibility that it is a reservoir not just of water, but of life.
"The Curiosity rover found mudstone, evidence of an ancient lake about 75km across, and the right pH for habitability", says Coates.
Mars is now cold, barren and dry but used to be warm and wet. It's less powerful in some ways, although if there were a pool of pure water, it is possible that its instruments could find it.
He said: "Magnesium, calcium, and sodium could be dissolved in the water to form a brine". "It's the kind of signal we would expect for liquid water".
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Antarctica has salty lakes, including Don Juan Pond, which is 18-times saltier than seawater, Priscu said. "It may exceed the salt content that any terrestrial organisms that we know of can survive in", he said.
Still, she said, there are microbes on Earth that have been able to adapt to environments like that.
The team gathered data using a sophisticated radar sensor known as MARSIS aboard the Mars Express spacecraft. The 200 km square study area is indicated in the left-hand image and the radar footprints on the surface are indicated in the middle image for multiple orbits.
Several years into the mission, MARSIS scientists began to see small, bright echoes under the south polar ice cap-so bright that the reflection could indicate not just rock underlying the ice, but liquid water. "On Mars, that's much more hard, of course, because we can't really drill into the ice". Its ground-penetrating radar detects boundaries between structures of different dielectric permittivity - a measure of a material's electrical polarisation under the influence of an external electric field. In other words, even the lowest likely value was above the highest value we've observed previously. With this, radar pulses were sent outward, ultimately bouncing back to the spacecraft. That difference in time gives scientists a clue about what type of matter the radar encountered.
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